The present invention relates to a radiation image conversion panel and in particular to a radiation image conversion panel exhibiting enhanced luminance and improved sharpness.
In recent years, radiographic imaging methods have been employed using a radiation image conversion panel employing photo-stimulable phosphors (hereinafter also denoted simply as stimulable phosphors). For example, as disclosed in U.S. Pat. No. 3,859,527 and JP-A No. 55-12144 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication), there is a radiation image conversion panel having a stimulable phosphor layer on the support. The stimulable phosphor layer of the radiation image conversion panel is exposed to radiation rays having passed through respective portions of the object to accumulate radiation energy corresponding to radiation ray transmittance of the respective portion of the object in the stimulable phosphor layer to form latent images (accumulated images) and scanning the stimulable phosphor layer with stimulating light (laser lights are usually used) causes the accumulated radiation ray energy to be radiated to emit light, the intensities of which are read to forming images. The thus formed images may be reproduced on various displays such as CRT or reproduced in the form of a hard copy.
The stimulable phosphor layer of the radiation image conversion panel used in the radiation image conversion method requires enhanced radiation absorption efficiency, enhanced light conversion efficiency, superior image graininess and high sharpness.
To enhance sensitivity to radiation, it is necessary to increase the thickness of the stimulable phosphor layer. However, an excessively thick layer often causes a phenomenon in which stimulated emission light is scattered between phosphor grains, preventing emission from the layer. With regard to sharpness, a thinner phosphor layer results in enhanced sharpness but an excessively thin layer leads to reduced sensitivity.
Image graininess, in general, depends on local fluctuation in radiation quantum number (so-called quantum mottle) or structural disorder of the stimulable phosphor layer of the radiation image conversion panel (so-call structure mottle). Decreasing the phosphor layer thickness results in a decreased number of radiation quantum to increase the mottle or leads to markedly increased structural disorder to cause the structure mottle to increase, forming deteriorated images. Accordingly, a thinner phosphor layer is needed to enhance image graininess.
As described above, image quality and sensitivity in radiation image conversion methods using the radiation image conversion panel are dependent on various factors. There have been made various studies to achieve improvements in sensitivity and image quality by adjusting plural factors relating to the sensitivity and image quality. Of these, an attempt in controlling the form of stimulable phosphor grains to enhance sensitivity and image quality was made as a means for improving sharpness of radiographic images. For example, JP-A No. 61-142497 discloses a method of using a stimulable phosphor layer comprising a fine columnar block which has been formed by sedimentation of a stimulable phosphor on a support having fine protruded patterns; JP-A 62-39737 discloses a method of using a radiation image conversion panel having a stimulable phosphor layer having a pseudo-columnar form which has been formed by producing cracks on the layer surface side; JP-A 62-110200 proposes a method in which a stimulable phosphor layer having voids is formed by vapor deposition onto the upper surface of a support, followed by growing voids by subjecting a heating treatment to produce cracks.
JP-A No. 2-58000 proposed a radiation image conversion panel having a stimulable phosphor layer, in which long and thin columnar crystals were formed with an incline at a given angle toward the direction normal to the support.
In the foregoing attempts to control the stimulable phosphor layer form, it was intended to enhance image quality by allowing the phosphor layer to have a columnar crystal structure. It was supposed that the columnar form prevented traverse diffusion of stimulated emission light (or photo-stimulated luminescence), i.e., the light reached the support surface with repeating reflection at the interface of cracks (or columnar crystals), thereby leading to markedly enhanced sharpness of images formed by the stimulated luminescence.
However, enhanced image quality is still desired even in radiation image conversion panels having the stimulable phosphor layer which has been formed by the foregoing vapor-phase growth (deposition) and in which the relationship between luminance and sharpness has not achieved sufficient characteristics.
There have been made an attempt to improve image quality, specifically, sharpness in radiation image conversion panel having a stimulable phosphor layer which was formed through gas phase growth (deposition), as described in JP-A No. 1-131498. This was achieved by the combination of a phosphor layer comprised of columnar stimulable phosphor crystals described above and a low refractive layer, thereby preventing reflection or refraction at the interface between layers and leading to enhanced image quality.
At the reading stage in the radiation image recording and reproducing method, stimulating light is irradiated onto one surface side of the radiation image conversion panel, stimulated luminescence emitted from phosphor particles (stimulated emission) is taken out by means of a light-receiving guide provided on stimulating light-irradiating side and read through photoelectric conversion. A method of receiving stimulated luminescence from both sides of the radiation image conversion panel (both-side light-concentrating and reading method) is also employed in cases when desired to take out stimulated luminescence emitted for stimulable phosphor particles as much as possible or in cases where radiation energy accumulation image formed in the stimulable phosphor layer varies in the direction of depth of the phosphor layer with respect to energy intensity distribution and it is intended to obtain the variation of the energy intensity distribution as image information. The both-side light-concentrating and reading method is disclosed, for example, in JP-A No. 55-87970.
Superiority of a radiation image conversion system employing radiation image conversion panels is greatly dependent on stimulated emission luminance (also called sensitivity) and sharpness of the obtained image, which are known to be affected by characteristics of the stimulable phosphor.
However, the foregoing radiation image conversion panels having the stimulable phosphor layer which has been formed by the foregoing vapor-phase growth (deposition) have not achieved enhanced luminance and improved sharpness as desired on the market and further improvements are required.
The present invention has been made in light of the foregoing. It is an object of the invention to provide a radiation image conversion panel exhibiting enhanced luminance and superior sharpness.
The present invention has been accomplished by the following constitution:
1. A radiation image conversion panel comprising a support having thereon a stimulable phosphor layer comprising columnar stimulable phosphor crystals, wherein the columnar crystals have a tip having an angle of 20xc2x0 to 80xc2x0 between a centerline in the direction of crystal growth and a line tangent to a section of the tip along the centerline;
2. The radiation image conversion panel described in 1., wherein the columnar crystals have an average diameter of 0.5 to 50 xcexcm;
3. The radiation image conversion panel described in 1. or 2., wherein the stimulable phosphor layer comprises a stimulable phosphor represented by the following formula (1)
M1X.aM2Xxe2x80x22.bM3Xxe2x80x33:eA xe2x80x83xe2x80x83formula (1)
wherein M1 represents an alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; M2 represents a divalent metal selected from the group consisting of Be, Mg, Ca, Sr, Ba, Zn, Cd, Cu and Ni; M3 represents a trivalent metal selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga and In; X, Xxe2x80x2 and Xxe2x80x3 each represent a halogen selected from the group consisting of F, Cl, Br and I; A represents a metal selected from the group consisting of Eu, Tb, In, Ga, Cs, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, Gd, Lu, Sm, Y, Ti, Na, Ag, Cu and Mg; a, b and e are each 0xe2x89xa6a less than 0.5, 0xe2x89xa6b less than 0.5 and 0 less than exe2x89xa60.2;
4. The radiation image conversion panel described in 3., wherein in formula (1), M1 is an alkali metal selected from the group consisting of K, Rb and Cs;
5. The radiation image conversion panel described in 3. or 4., wherein in formula (1), X is Br or I;
6. The radiation image conversion panel described in any of 3., 4. and 5., wherein in formula (1), M2 represents a divalent metal selected from the group consisting of Be, Mg, Ca, Sr and Ba;
7. The radiation image conversion panel described in any of 3. through 6., wherein in formula (1), M3 represents a trivalent metal selected from the group consisting of Y, Ce, Sm, Eu, Al, La, Gd, Lu, Ga and In;
8. The radiation image conversion panel described in any of 3. through 7., wherein in formula (1), 0xe2x89xa6bxe2x89xa610xe2x88x922;
9. The radiation image conversion panel described in any of 3. through 8., wherein in formula (1), A is a metal selected from the group consisting of Eu, Cs, Sm, Tl and Na;
10. The radiation image conversion panel described in any of 3. through 9., wherein the stimulable phosphor represented by formula (1) is a stimulable phosphor represented by the following formula (2):
CsX:A xe2x80x83xe2x80x83formula (2)
wherein X represents Br or I; A represents Eu, In, Tb or Ce.
11. The radiation image conversion panel described in 1., wherein the stimulable phosphor layer is formed by a vapor deposition process.
12. A reading apparatus using the radiation image conversion panel described in any of the foregoing 1. through 10., wherein the stimulable phosphor layer of the radiation image conversion panel is exposed to a laser beam having a diameter of not more than 100 xcexcm.